Recent concern for the welfare of the world population has included consideration of additional means for feeding the rapidly increasing number of people involved. The problem embraces providing both adequate per capita caloric intake and a balanced diet, with particular reference to the acknowledged lack of sufficient protein-affording foods in many parts of the world. One means for providing necessary protein supplies is through the growth of single-cell protein-affording microorganisms, such as yeast, bacteria and algae, for use as either foods or food supplements.
Production of single-cell protein (SCP) materials in large quantity may be accomplished by fermentation processes employing, for example, carbohydrate, hydrocarbon or oxygenated hydrocarbon materials as substrate. Principal requirements are that the substrate material be inexpensive and readily consumed by the selected microorganism so that process costs are not excessive. Equally important is the acceptability and utility of the SCP material, including yeasts, as a food or food component. The latter considerations include taste and odor factors relating to public acceptance as well as metabolic and toxicity factors relating to suitability of SCP material for inclusion in the human diet.
Both the technical and the patent literature describe fermentation processes for production of microorganisms which readily afford useful SCP materials. For example, yeasts have been grown on the polysaccharides contained in waste sulfite liquor and on the normal alkane components of a gas oil hydrocarbon fuel. Production of bacteria has been similarly described as, for example, in U.S. Pat. No. 3,546,071, which employs a mixture of oxygenated hydrocarbons, including ethanol, as substrate. Fermentation to produce yeasts or bacteria comprises an oxidation process, evolving much heat and requiring both substantial oxygen transfer and good control of fermentation temperature. Preferred substrate materials will already contain as much combined oxygen as possible in order to minimize the heat release and the oxygen requirement. Production of food-grade SCP material may also require an extraction step to limit the presence of undesirable, residual substrate material such as high-molecular-weight hydrocarbons or slowly fermented oxygenated hydrocarbon species.
Most of the fermentation processes planned or in use currently for production of SCP material are intended to provide primarily an animal feed supplement and hence to supply protein for human consumption only indirectly. However, certain microorganisms, notably yeasts within the Saccharomycetoideae and Cryptococcoideae sub-families, have been certified by the Food and Drug Administration for direct use in foods intended for human consumption.
One highly desirable substrate material is ethanol. It exhibits complete water solubility, is already in a partially-oxidized state, is itself acceptable for use in foods, and creates no problem as to removal from the produced microorganism cells. However, ethanol is a growth inhibitor to many microorganisms and some others do not grow well in its presence.
Other alcohols are also suitable substrates with certain yeasts. For example, French Pat. No. 2,006,235 describes the preparation of amino acids by the growth of various yeast strains on methanol.
The economics of SCP production require that the substrate material be relatively inexpensive. In comparison with waste materials employed as substrates in many commercial fermentations, ethanol is sufficiently expensive to require that it be used most efficiently if selected to serve as a substrate.